Whether you think veganism is optimal or not, no one should have Child services threatened on them because they raise their child in veganism. The human body is adaptable there are things available to provide the necessary nutrients. Spirilina, for example, is a vegetarian source of vitamin b12 and the best source of protein out there. Add some omega 3's for good fats and you're all set.
There are kids out there living with all kinds of true abuse. My wife has a small scar on her arm from her father having thrown a knife at her when she was a kid. I suspect a caring vegan family is in a far different league from a family with an abuse parent that does stuff like that. It really gets my hackles up someone should have the threat of a child being taken away for simply a defensible eating choice. Amazing.